An original double-masked translation priming study investigates how trilingual translation trainees process their non-dominant languages (L2 and L3) and how these languages influence one another. We recruited 24 French (L1)- English (L2)- Spanish (L3) unbalanced trilinguals to perform lexical decision tasks in their L2 and L3. Target words were preceded by two primes, which were either the same word (repetition), a translation in one language, translations in two languages or unrelated words (in one or two languages). The results highlighted strong translation priming effects, with a repetition effect in both target languages. In addition, when the translation primes belonged to the other non-dominant language, reaction times (RTs) were slower in comparison to semantically unrelated primes in the same priming language. When two different languages were presented as a prime, L1 primes were more efficient when presented as first prime. These results are in line with previous experiments on masked translation priming studies in trilinguals and suggest that the multilingual lexicon is mediated by the L1.